Just think of it as a very large starter. Some people scoop out up to 2/3 of the cake so as to not overpitch, but in homebrew settings everpitching is not really an issue. I've poured onto a yeast cake many times before and it works great. Fermentation time frame is the same.

I try to rack the first beer off the yeast as close to pitching time as possible. Generally, while the new beer is chilling I will rack the old beer to a secondary fermenter. Then I just dump in the new wort and aerate.

When I do this I try to keep the first beer at 1.050 or less (1.040 is better), and the next beer is at least 1.080 or so. If I'm pitching right on the cake, then I try to make sure the original beer is a similar style to the beer going on top of it. (i.e., APA to IIPA, a small Dubbel to Quad, Mild to Barleywine, etc.)

You need to take sanitary precautions, but you are pitching sanitary wort onto yeast. The environment the wort is going into should be sanitary, you just have to make sure you arent introducing anything wild while transferring. If you are doing it pretty quickly and covering the yeast in between transfers you should be good to go.

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Michael P MitchemBeer and Ale Research Foundation (B.A.R.F.)AHA Member since 2011

Dumb question from a newb here, but if you pour wort on an existing cake, doesn't that mess with your sanitation?

I don't see it any different than re-pitching yeast. Your fermenter should have been sanitized the first time so the real danger is introducing bacteria when you open it to siphon out or add the new wort. What I will do, and its working for me, is to brew a 5 gal batch of beer and then split that yeast into a new bucket so that I have 1/2 of the yeast as a "re-pitch" and 1/2 directly on the old cake. I only do this once because I do have sanitation concerns so I don't continually re-use the same cake without a clean out. I do think that you will have some over-pitching concerns if you use the whole cake for an ale. And yes, it's gonna take off like a rocket so be ready with a blow off tube if you are using a carboy. For a lager, I will put the wort on 100% of the old cake because I've been told you can't really over pitch a lager.

The thing with washing yeast is that it is really not "washing", it's rinsing and you won't remove any bacteria. In fact, if you're not careful you can also introduce bacteria just like anytime you have the yeast open to the air. I've never done a true acidic wash and I don't think too many people do.

Sanitation is not much of a concern, assuming the beer you had in there previously was sanitary. Over pitching definitely is. Also dead yeast cells are a concern. Also the braun hefe on the side of the carboy is a concern because it has some bitter and nasty flavor compounds you probably don't want in your beer. Left over hop compounds might be an issue as well.

Unless you are going from a very low gravity beer to a high gravity beer (ordinary bitter to barley wine) I think you are much better off collecting the yeast slurry, using the pitching calc at www.mrmalty.com and approximating an appropriate pitch from there. You are better off pitching "the right" amount of yeast than overpitching, everytime.

I always harvest the yeast and wash and sanitize the container - but I collect and repitch multiple times, so I don't want a whole slurry in there. I overpitched a light ale once and it was highly estery with US-05 slurry. I seem to recall that if the yeast don't reproduce sufficiently, they instead throw off acetaldehydes or phenols, because their full reproduction cycle has been short circuited. I will defer to the science guys on that, but it makes sense to at least remove half of the yeast cake if repitching to a comparable or smaller beer.

I always harvest the yeast and wash and sanitize the container - but I collect and repitch multiple times, so I don't want a whole slurry in there. I overpitched a light ale once and it was highly estery with US-05 slurry. I seem to recall that if the yeast don't reproduce sufficiently, they instead throw off acetaldehydes or phenols, because their full reproduction cycle has been short circuited. I will defer to the science guys on that, but it makes sense to at least remove half of the yeast cake if repitching to a comparable or smaller beer.

I always harvest the yeast and wash and sanitize the container - but I collect and repitch multiple times, so I don't want a whole slurry in there. I overpitched a light ale once and it was highly estery with US-05 slurry. I seem to recall that if the yeast don't reproduce sufficiently, they instead throw off acetaldehydes or phenols, because their full reproduction cycle has been short circuited. I will defer to the science guys on that, but it makes sense to at least remove half of the yeast cake if repitching to a comparable or smaller beer.